This project has focused on the function of immune response (Ir) genes in the activation of thymus-derived (T) lymphocytes. At present we have demonstrated genetically that Ir genes control murine T-lymphocyte proliferative responses and that antibodies directed against I region products (anti-Ia) antibodies can inhibit these responses. Furthermore, we have shown that antigen presenting cells from low responder mice specifically fail to stimulate proliferation to the antigen under Ir gene control in primed (responder x nonresponder F1) T lymphocytes. In the dual Ir gene controlled system of the response to GL phi, neither parental spleen cell population could present the antigen to GL phi primed F1 responder T lymphocytes. These results have led to the conclusion that one site of expression for Ir gene products is in the antigen presenting cell. BIBLIOGRAPHIC REFERENCES: Schwartz, R.H., Horton, C.L., and Paul, W.E.: T-lymphocyte-enriched murine peritoneal exudate cells. IV. Genetic control of cross-stimulation at the T-cell level. J. Exp. Med. 145: 327-343, 1977. Schwartz, B.D., Kask, A.M., Sharrow, S.O., David, C.S., and Schwartz, R.H.: Partial chemical characterization of Ia antigens derived from murine thymocytes. Proc. Natl. Acad. Sci. 74: 1195-1199, 1977.